When perusing printed text, a reader can quickly skip
portions of the document, reading only those sections that are of
interest. Typeset documents allow such structured browsing by using
layout cues to present the underlying document structure; from here,
the eye's ability to ``randomly'' access portions of the
two-dimensional printed page appears to take over.
The passive information in
a printed document is accessed by an active reader
capable of selectively perusing the text.
Hence, visual documents themselves need not be interactive.

Things are different with audio. This passive-active relationship is
reversed in traditional oral communication; the information flows past
a passive listener who has little control on what is heard. The
problem is particularly severe when presenting structured information
(e.g., complex mathematics) -a listener is likely to lose
interest by the time the relevant information is presented. Hence, we
need to enable active listening, i.e., enable the listener to determine
what is heard. Therefore, to be effective, audio documents need to be
interactive.

The first step is to make audio documents interactive. Techniques for
specifying and modifying how particular objects are rendered were
described in s:rules_and_styles. In addition, a browser for
audio documents allows a user to interactively traverse the internal
high-level representation described in c:recognition and listen
to portions of interest. The browser provides basic tree-traversal
commands. These can be composed to effectively browse the information
structure.

The design of our browser is motivated by the conjecture that most of
visual browsing actions are directed by the underlying structure
present in the information. Thus, when we read a complex mathematical
expression that involves a fraction, we can quickly look at the
numerator while reading the denominator. This single action of
looking up at the numerator can be decomposed into a series of atomic
tree traversal movements with respect to the structure of the
expression. In the visual context, these actions happen extremely
fast, leading to a feeling that the eye can access relevant portions
of the visual display almost randomly. However, this notion of
randomness disappears when we consider that such visual browsing
becomes difficult in a badly formatted document where the underlying
structure is not so apparent. Similarly, even when presented with a
well-formatted document, a person unfamiliar with the subject matter
finds it impossible to perform the same kind of visual browsing.
Visual browsing thus depends on familiarity with the underlying
structure and a clear rendering of this structure.
AsTeR parallels this functionality by building up a rich internal
representation and providing a set of atomic actions to traverse this
representation. The effectiveness with which a user can browse this
representation is now a function of the user's familiarity with the
structure in the subject matter being presented.

We present the browser as follows: s:browse-motivation
motivates the need for a browser by analyzing how visual
browsing works. Based on this, we derive a corresponding model for
audio browsing. We identify a set of atomic browsing actions that
enable general browsing. s:traverse-structure describes how a
user can traverse the high-level representation of a document. This
section introduces the concept of a current selection and
describes how the user is unobtrusively cued to the nature of the
current selection. s:read-selection describes how the listener
can execute actions after setting the current selection. These actions
include listening to the current selection, rendering it relative to
its parent, and listening to the rest of the document.
Cross-references form an important component of technical documents
and are described in s:cross-references. A
particularly difficult problem faced when listening to mathematical
texts on conventional talking books, or even when reading printed
mathematical texts, is keeping track of equation numbers and
understanding statements that refer to equations and theorems by their
numbers in the running text. We describe a flexible mechanism that
allows a listener to annotate cross-referenceable objects with
meaningful labels that can be used to refer to such objects in later
cross-references. This section also describes how places of interest in a
document can be marked using a bookmark facility.
a:browser documents the external interface to the browser.
The browser, along with the ability to change rendering rules and
styles,
makes audio documents produced by AsTeR fully interactive.